linux-kernel - Re: [PATCH RFC 1/6] dcache: sweep cached negative dentries to the end of list of siblings

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Date:   Thu, 15 Apr 2021 16:50:56 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Gautham Ananthakrishna <gautham.ananthakrishna@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, matthew.wilcox@...cle.com,
        khlebnikov@...dex-team.ru
Subject: Re: [PATCH RFC 1/6] dcache: sweep cached negative dentries to the
 end of list of siblings

On Wed, Apr 14, 2021 at 03:00:48AM +0000, Al Viro wrote:

> Ugh...  So when dput() drives the refcount down to 0 you hit lock_parent()
> and only then bother to check if the sucker had been negative in the first
                                              ^^^^^^^^^^^^^^^^^
                                              had zero refcount, of course.
> place?

> > @@ -1970,6 +2021,8 @@ void d_instantiate(struct dentry *entry, struct inode * inode)
> >  {
> >  	BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
> >  	if (inode) {
> > +		if (d_is_tail_negative(entry))
> > +			recycle_negative(entry);
> >  		security_d_instantiate(entry, inode);
> >  		spin_lock(&inode->i_lock);
> >  		__d_instantiate(entry, inode);
> 
> Wait a bloody minute.  What about d_instantiate_new() right next to it?

Another fun question: where's the proof that __d_add(dentry, non_NULL_inode)
won't happen to dentry marked tail-negative?  From a quick grep I see at
least one such place - on success cifs_do_create() does
        d_drop(direntry);
	d_add(direntry, newinode);
and it would bloody well evade what you are doing in d_instantiate().
Same seems to be true for nfs_link()...